Purification of used frying oil by supercritical carbon dioxide extraction

Supercritical fluid extraction (SFE) was used for the purification of used frying oil at different temperatures (40 and 60°C) and pressures (150, 225 and 300 bar). The calculated solubilities of the oil ranged from 0.3×10−3 to 7.4×10−3 g oil/g CO2 depending on the extraction conditions. The solubility and the selectivity of triglycerides (TG) were between those of the low molecular weight compounds (LMWC) and the polymers. The calculated relative separation efficiency (R) of low molecular weight compounds (LMWC) from TG was highest under the low pressure and high temperature extraction conditions, but the extraction rate was minimal under this condition. However, the separation efficiency (R) of polymers from TG was not greatly dependent on the pressure and temperature. Based on the relative separation efficiency result, two-stage supercritical carbon dioxide extraction was performed for the effective purification of TG from used frying oil. LMWC were removed under the low pressure extraction conditions (150 bar, 60°C) of the first stage extraction, and then TG were concentrated under the high pressure extraction conditions of the second stage extraction (300 bar, 60°C). The extracts obtained from the second to the fifth fraction of the second stage included less than 1% polymer, more than 92% TG, and less than 8% LMWC. The conjugated diene contents of these extracts ranged from 0.202 to 0.231%, showing a similar conjugated diene content to the fresh frying oil. The present data clearly showed that two-stage supercritical CO2 extraction was an efficient method for the purification of TG from used frying oil.